Influence of rostral fluid shift on upper airway size and mucosal water content

Abstract

Study objective: Fluid displacement from the legs during recumbency while in bed might narrow the upper airway (UA) in association with nuchal fluid accumulation that may contribute to the pathogenesis of obstructive sleep apnea (OSA). The aim of this study was to test the hypothesis that rostral fluid displacement from the legs causes a greater decrease in UA cross-sectional area (UA-XSA) and a greater increase in UA mucosal water content (UA-MWC) and internal jugular venous volume (IJVVol) in subjects with OSA than in those without OSA.

Methods: Subjects underwent baseline assessment of leg fluid volume (LFV) measured by bio-electrical impedance, as well as UA-XSA and UA-MWC by magnetic resonance imaging. They were then randomly assigned to a 20-min period either with or without application of lower body positive pressure (LBPP) of 40 mm Hg, followed by a 15-min washout period, after which they crossed over to the other arm of the study. Measurements of LFV, UA-MWC, and UA-XSA were repeated after each arm of the study.

Results: In 12 subjects without sleep apnea, UA-XSA increased and UA-MWC decreased significantly, whereas in 12 subjects with OSA, UA-XSA decreased and UA-MWC increased significantly in response to LBPP. The changes in UA-XSA and UA-MWC in response to LBPP differed significantly between the 2 groups (p = 0.006 and p < 0.001, respectively), despite similar changes in LFV and IJVVol.

Conclusions: Our results suggest that rostral fluid shift may contribute to the pathogenesis of OSA at least partly through narrowing of the UA due to transudation of fluid into the UA mucosa.

Keywords: fluid displacement; lower body positive airway pressure; obstructive sleep apnea; upper airway cross-sectional area; upper airway mucosal water.

Figures

Figure 1. Magnetic resonance images of the upper airway (UA) of subjects.

top: In the NSA subject, LBPP caused UA-XSA to increase compared to control (from 3.01 to 3.71 cm2). bottom: In the OSA subject, LBPP caused UA-XSA to decrease compared to control (from 2.04 to 1.72 cm2). LBPP, lower body positive pressure; NSA, no sleep apnea; OSA, obstructive sleep apnea; UA, upper airway; UA-XSA, upper airway cross-sectional area.

Figure 2. Comparisons of mean UA-XSA during control and LBPP in the NSA and OSA groups.

In the NSA group, mean UA-XSA during LBPP increased significantly compared to control (log-transformed UA-XSA, 0.60 ± 0.42 versus 0.51 ± 0.38, respectively [median (interquartile range); absolute UA-XSA, 1.67 (1.17) versus 1.74 (1.36) cm2, respectively], whereas in the OSA group mean UA-XSA decreased significantly during LBPP compared to control (log transformed UA-XSA, 0.18 ± 0.25 versus 0.26 ± 0.23, respectively) [median (interquartile range); absolute UA-XSA, 1.17 (0.52) versus 1.23 (0.40) cm2, respectively]. The change in mean UA-XSA in response to LBPP differed significantly between the 2 groups (p = 0.006). Note that data are plotted on a log scale. LBPP, lower body positive pressure; NSA, no sleep apnea; OSA, obstructive sleep apnea; UA-XSA, upper airway cross-sectional area.

Figure 3. Comparisons of UA-MWC during control and LBPP in both the NSA and OSA groups.

In the NSA group, log-transformed UA-MWC during LBPP decreased significantly compared to control (9.3 ± 0.6 versus 9.6 ± 0.5, respectively), whereas in the OSA group, log-transformed UA-MWC during LBPP increased significantly compared to control (9.6 ± 0.3 versus 9.5 ± 0.4, respectively). The change in UA-XSA in response to LBPP differed significantly between the 2 groups (p